1. Field of the Invention
This invention relates to an RC composite component provided with a spark gap for protection against overvoltage and more particularly it relates to improvements for adjusting discharge start voltage to a predetermined range.
2. Description of the Prior Art
RC composite components having a spark gap are used in lightning protectors for bypassing circuits included in television receivers, video tape recorders, business transceivers and the like to provide protection against abnormally high voltages.
FIG. 1 is a front view showing an example of a conventional RC composite component having a spark gap. FIG. 2 is a rear view of the component of FIG. 1. FIG. 3 is a section taken along the line III--III of FIG. 2, showing the component of FIG. 1 in a resin-clothed state. FIG. 4 is a diagrammatic view of the electric circuit arrangement of the device of FIG. 1.
The RC composite component with a spark gap includes a dielectric substrate 1 formed of ceramic or the like. One surface of the dielectric substrate 1, as shown in FIG. 1, is provided with two capacitor electrodes 2, a film resistor 3 interconnecting said capacitor electrodes 2, and discharge electrodes 4 each integrally extending from the respective capacitor electrodes 2. A spark gap g is defined between the discharge electrodes 4. The spark gap dimension is indicated by "a". Each capacitor electrode 2 has a lead wire 5 connected thereto as by soldering. The other surface of the dielectric substrate 1, as shown in FIG. 2, is formed with a common capacitor electrode 6. The arrangement thus made provides two series-connected capacitors C1 and C2, and a resistor R and a spark gap g connected in parallel with said series-connected capacitors C1 and C2, as shown in FIG. 4. The component is resin-coated with a resin 7 such as of the phenol type, as shown in FIG. 3. This coating is performed such that the resin 7 is not applied to the entire area of one surface of the dielectric substrate 1 in order that the spark gap g defined between the discharge electrodes 4 is not covered with the resin 7.
In the arrangement described above, if an abnormally high voltage appears across the lead wires 5, a conductive path is formed across the spark gap g, through which a discharge occurs, thus reducing the peak value of the abnormally high voltage and preventing damage to the component. At the end of the discharge, the conductive path is blocked and the spark gap is again in an insulated state.
In such RC composite components, it will be understood that in order to raise the flashover voltage at which a discharge across the discharge electrodes 4 begins, the dimension "a" may be increased. However, as the dimension "a" increases, the size of the entire component also increases, thus failing to meet requirements for miniaturization of components. In this connection, if a flashover voltage of 3.0 kV AC or above is to be obtained, the dimension "a" must be at least 6 mm. Thus, the desire to raise the discharge start voltage runs counter to the desire to miniaturize components.
When RC composite components with a spark gap are used, e.g., on the antenna terminal board of a television receiver, the upper and lower limits of discharge voltage are prescribed by the UL standards. For example, there are requirements that the lower limit of flashover voltage be selected so that no discharge will occur at or below 3.5 kV AC and that the upper limit be selected such that satisfactory discharge will occur as determined by a 5.0 kV discharge test according to the UL standards. In order to meet these requirements, it is insufficient only to raise the flashover voltage and it is necessary to set such flashover voltage in a predetermined range.
It has heretofore been practiced to apply a wax to the spark gap region, though not primarily intended to raise the flashover voltage. The primary object of such application of wax is as follows.
Even after the component has been covered with the resin 7, as shown in FIG. 3, the spark gap g defined between the discharge electrodes 4 remains exposed to allow satisfactory discharge. In such condition, the spark gap g can be easily influenced by moisture and other external factors. It is known that the flashover voltage varies relatively widely under the influence of moisture and other external factors. It has been found advantageous to apply a wax to the spark gap g for the purpose of reducing such variation as much as possible. Such application of wax is performed e.g., by immersing the entire component in a wax after it has been covered with the resin 7, as shown in FIG. 3. Thus, at least the spark gap region is covered with the wax and the variation of flashover voltage due to moisture and other external factors can be reduced. In this connection, it has also been found, as a matter of fact related to the present invention, that the flashover voltage more or less rises. Such rise in flashover voltage, however, is no more than a "by-product" and is insufficient to meet the requirements described above while miniaturizing components. It is true that the thicker the film of wax, the higher the flashover voltage, but it is difficult to control the film thickness because immersion process is employed for application of such wax.
On the other hand, varnishes are sometimes used to provide increased withstand voltage values for capacitors in general. Application of varnishes to capacitors is performed by immersing the completed capacitor in the varnish and then baking the same. This increases the creeping distance between the capacitor electrodes, thus providing an increased withstand voltage value. Provision of increased withstand voltage values for capacitors has been described as an example of a technical field in which varnishes are used, but varnishes have not been used for the purpose of increasing the flashover voltage across the spark gap of RC composite components having such spark gap. The reason in brief is that the increase of withstand voltage values for capacitors is essentially different in object from the increase of flashover voltage. The higher a withstand voltage value for capacitors, the more highly it is evaluated, putting aside the results obtained. On the other hand, the evaluation of rises in flashover voltage across the spark gap of RC composite components having such spark gap is not of such a nature that the higher the value, the better. That is, it is necessary to set the flashover voltage in a predetermined range. If a film of varnish on the spark gap region (discharge electrodes) is so thick that the flashover voltage is higher than the desired value, predetermined discharge will not occur, sometimes even resulting in damage to the component. Conversely, it is also undesirable that such film of varnish is so thin that a desired high discharge voltage cannot be obtained. For this reason, it is required that the thickness of a film of varnish formed on the spark gap (discharge electrodes) be controllable so as to set the flashover voltage in a predetermined range. However, the process of immersing RC composite components with a spark gap in a varnish tends to produce variation in the thickness of the varnish film and cannot be said to be a desirable process. Thus, the varnish immersion process heretofore employed in general capacitors cannot be readily employed in RC composite components with a spark gap because the intended object of the process differs.
Further, if said varnish immersion process is applied to RC composite components having a spark gap, the following drawbacks will result: First, since the varnish adhering also to the film resistor 3 has adverse influences on the electrical characteristics of the resistor. Secondly, if immersion in varnish is followed by coating with the resin 7, the force with which the resin 7 adheres to the substrate 1 is reduced so much that the resin coating may be damaged by voltage. This is because the reduced adhesion of the resin 7 shortens the creeping distance between the capacitor electrodes 2. Thirdly, the varnish adheres also to the lead wires 5 and contaminates the latter, thus detracting from solderbility.